mkfs.f2fs: discard obsolete blocks to avoid roll-forward recovery
mkfs.f2fs: discard obsolete blocks to avoid roll-forward recovery

If there are many obsolete blocks written by previous usage, we can recover
wrong blocks with them.
So, mkfs should discard whole wrong information correctly by removing possible
same checkpoint versions.

Change-Id: Ic7a4d88e985e93056a225a586396a75f8dee2a69
Signed-off-by: Jaegeuk Kim <71255387a32c360dfbe6dfb7aaeb123bf1dc8cb6@kernel.org>

TeamTwisted/android_external_f2fs-tools,TeamTwisted/android_external_f2fs-tools